The invention relates to a new method for the preparation of N-alkyl-Nxe2x80x2-nitroguanidine.
A method for the preparation of N-methyl-Nxe2x80x2-nitroguanidine, in which 5-methylisothiuronium sulphate is nitrated in the usual manner and in which, subsequently in a second reaction stage, the methyl mercapto-group is substituted by methylamine, is described in JACS 76,1877 (1954). The disadvantage of this is that it is a two-stage conversion. Whilst the yields in the two stages are very good, the splitting-off of methylmercaptan, above all when carried out on a commercial scale, raises problems in terms of process engineering.
It is known, furthermore, that N-methyl-Nxe2x80x2-nitroguanidine can be obtained by reacting an alkaline solution (potassium hydroxide) of nitroguanidine with methylamine hydrochloride at 60xc2x0 C. (JACS 69,3028 (1947)). In order to remove inorganic impurities, in the case of this method it is necessary to provide up to two recrystallizations. As a result, yield losses cannot be avoided.
A method for the preparation of N-methyl-Nxe2x80x2-nitroguanidine, in which nitroguanidine is reacted with aqueous methylamine solution at temperatures between 0xc2x0 C. and 40xc2x0 C., is described in DE-A-19 61 16 54. What is disadvantageous about this method is the fact that long reaction times of 24 hours result on account of the low reaction temperatures.
An object of the present invention has been to provide a method that enables good yields of N-alkyl-Nxe2x80x2-nitroguanidine to be produced under conditions that are as selective as possible and with clearly reduced reaction times. Surprisingly, it has been possible to achieve this object by means of a method that has the features of the main claim. Preferred embodiments are characterised by the subclaims.
In accordance with the invention metered doses of an alkylamine solution are added to a suspension of nitroguanidine at temperatures  greater than 60xc2x0 C., preferably at temperatures of 60 to 90xc2x0 C., in particular preferably at temperatures of 60 to 75xc2x0 C. In this connection, by alkylamine is to be understood, a branched or non-branched low alkylamine with 1 to 6 C-atoms, benzylamine or cyclohexylamine, preferably a hexyl-, pentyl-, tert.-butyl, butyl-, isopropyl-, propyl-, ethyl or methylamine, in particular preferably methylamine. Metered additions of a 40% methylamine solution are preferably added. In this connection, the equilibrium is shifted in the direction of the reaction end product by removing, preferably expelling, the ammonia that is formed. Advantageously, this happens in accordance with the invention by passing through air, nitrogen, CO2 or other suitable gases. The ammonia that is formed can also be captured by adding acid, for example nitric acid, or dry ice (CO2). The pH-value should then, as far as possible, be xe2x89xa79. By conducting this reaction it becomes possible for the alkylamine to be reacted directly to completion. Surprisingly, in this way a reaction can be conducted at substantially higher temperatures. The result is a substantial shortening of the reaction time in comparison with DE-A-196 11 654. The reaction will already have come to an end after 15 to 20 minutes. After cooling, the N-alkyl-Nxe2x80x2-nitroguanidine, for example the N-methyl-Nxe2x80x2-nitroguanidine, that is formed with a high level of purity is filtered off, washed and dried. In order to prevent needles from forming during the cooling process, crystal-influencing additives, for example polyvinyl alcohol, can be added.
Alternatively, the nitroguanidine-nitrate can also be used as a starting material instead of the free nitroguanidine. The acid that is formed in this connection as a result of hydrolysis is preferably neutralized to a pH-value xe2x89xa79 before the addition of alkylamine.
In addition to employing the discontinuous method, it is also possible to prepare the N-alkyl-Nxe2x80x2-nitroguanidine, preferably the N-methyl-Nxe2x80x2-nitroguanidine, in a continuous method. In this connection, aqueous alkylamine solution, with the alkylamines being defined as above, preferably methylamine solution is added at temperatures  greater than 60xc2x0 C., is added to a stirred nitroguanidine suspension in a first reservoir. The constituents are caused to react to completion in a reservoir downstream thereof before the reaction solution is cooled in a third boiler, and the N-alkyl-Nxe2x80x2-nitroguanidine that is formed, preferably the N-methyl-Nxe2x80x2-nitroguanidine is filtered off. Also with this continuous method is the equilibrium shifted in the direction of the reaction end product by removing the ammonia that is formed, as described above. Both the N-alkyl-Nxe2x80x2-nitroguanidine that is formed in the case of the discontinuous method and the N-alkyl-Nxe2x80x2-nitroguanidine that is formed in the case of the continuous method can, if applicable, be recrystallized. The melting point of the N-methyl-Nxe2x80x2-nitroguanidine obtained in this way in accordance with the invention amounts to 159 to 161xc2x0 C.
The starting materials that are used, nitroguanidine and the alkylamines, for example the methylamine, are generally known compounds of organic chemistry. The method in accordance with the invention is, as a rule, carried out in an aqueous suspension. It is also possible, however, to operate in organic/aqueous systems, in which case all the usual organic solvents that are mixable with water can be used. Ketones, such as acetone, methyl-ethyl-ketone or methyl-isobutyl-ketone, nitriles, such as acetonitrile or proprionitrile, and also alcohols, such as methanol or ethanol, may be mentioned by way of example.